Whodunit? Or rather Whatdunit? Or Whatizit?

How does something buried inside an ice ball only 311 miles wide, provide the pop to propel a plume 600 miles out of the moon's south pole? "The biggest puzzle with Enceladus is where is the heat source," says Cassini scientist Linda Spilker of NASA's Jet Propulsion Laboratory, which manages the mission. "This tiny moon 'should' be frozen over like the others orbiting Saturn."

And there is one even more compelling question.

"Is it possible for life to exist on Enceladus, the tiny icy satellite of Saturn," asked planetary scientists Susan Kieffer of the University of Illinois and Bruce Jakosky of the University of Colorado, in the journal Science last year. Life on Enceladus, hidden in an interior lake or ocean suspected under its ice, has consumed planetary researchers since 2005, when Cassini first spotted the plume.

In the latest flyby, its seventh, the spacecraft passed within 62 miles of the moon's surface. The gravitational pull of the giant planet Saturn creates tides within the tiny moon that seem to drive the plumes, perhaps opening and closing rifts to vent geysers, Spilker says, "but we're not sure exactly how."

Indeed, geophysicists have conjured a plethora of competing explanations for geyser rumbling from the belly of Enceladus; models with names like "Cold Faithful" and "Frigid Faithful" in homage to Yellowstone's Old Faithful geyser. The debate centers largely on whether a (perhaps life-sheltering) sea supplies the plumes, or if they simply result from icy rift walls grinding together under the pull of Saturn. "We're more confident from heat signatures that liquid water exists down there, but the question isn't settled," Spilker says.

A "radiolytic" model, proposed by John Cooper of NASA's Goddard Space Flight Center in Greenbelt, Md., and colleagues in the current Planetary and Space Science journal, suggests Saturn may be essentially frying the moon with radiation. "Over billions of years," they suggest, radiation slowly cooked ammonia, methane and hydrogen peroxide within Enceladus, building up pressure for "continuous but variable high activity" over the last 100 million years or so.

The last idea might explain why Enceladus hasn't essentially blasted itself apart by now. At the present rate of plume activity, say Cooper and colleagues, the moon should have lost the bottom quarter of the planet over the lifetime of the moon, instead of the ridges observed on the south pole only several hundred feet deep.

"We happen to see the phenomenon now at Enceladus," says the study, but the radiolytic model might also explain signs of ice plumes on Neptune's moon Triton, and the dwarf planets Eris and Haumea located beyond, Pluto, or "any icy body in a radiation environment."

But wait, there's more. "Finally, our proposal of the radiolytic cryovolcanism model for Enceladus has direct implications for astrobiological potential of this active moon," says the study. Alien space microbes are more likely to thrive in hot springs on Enceladus, the study argues, than on Europa or Ganymede, Jupiter's highly-irradiated, but geyser-less moons.

Those are fighting words in the planet science community, with NASA recently deciding to send a "Europa Jupiter System Mission" to the Jovian system ahead of another mission to Saturn.

But until something does land on Enceladus, we'll just have to wait for the next Cassini flyby, on Nov. 21, when the spacecraft buzzes Enceladus once more, this time at 998 miles altitude, in a bid to gain broad images of the "tiger stripe" ridges on the moon's south pole from which the geysers vent. "the more times we fly by, the better our chances understanding just what is going on down there," Spilker says.

Enlarge By NASA A photo of Enceladus, Saturn's moon, taken by the Cassini spacecraft on March 12, 2009.